Heroin addiction kills 30,000 people per year in Russia – a third of global deaths from the drug – but now there is the added problem of krokodil. He estimates that overall, around 5 per cent of Russian drug users are on krokodil and other home-made drugs

In the city's main drug treatment centre, Artyom Yegorov talks of the devastation that krokodil is causing. "Desomorphine causes the strongest levels of addiction, and is the hardest to cure," says the young doctor, sitting in a treatment room in the scruffy clinic, below a picture of Hugh Laurie as Dr House.

"With heroin withdrawal, the main symptoms last for five to 10 days. After that there is still a big danger of relapse but the physical pain will be gone. With krokodil, the pain can last up to a month, and it's unbearable. They have to be injected with extremely strong tranquilisers just to keep them from passing out from the pain."

While heroin costs from £20 to £60 per dose, desomorphine can be "cooked" from codeine-based headache pills that cost £2 per pack, and other household ingredients available cheaply from the markets.

It is a drug for the poor, and its effects are horrific. It was given its reptilian name because its poisonous ingredients quickly turn the skin scaly (krokodil=crocodile). Worse follows. Oleg and Sasha have not been using for long, but Oleg has rotting sores on the back of his neck.

"If you miss the vein, that's an abscess straight away," says Sasha. Essentially, they are injecting poison directly into their flesh. One of their friends, in a neighbouring apartment block, is further down the line.

"She won't go to hospital, she just keeps injecting. Her flesh is falling off and she can hardly move anymore," says Sasha. Photographs of late-stage krokodil addicts are disturbing in the extreme. Flesh goes grey and peels away to leave bones exposed. People literally rot to death.